185/70R15 vs 175/75R15
Going from one to the other changes the overall height by +0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/70R15 | 175/75R15 | Change |
|---|---|---|---|
| Overall height | 25.2" | 25.3" | +0.5 % |
| Sidewall | 5.10" | 5.17" | +1.4 % |
| Section width | 185 mm | 175 mm | -5.4 % |
| Circumference | 79.2" | 79.6" | +0.5 % |
| Revs per mile | 800 | 796 | -0.5 % |
| Axle height | 12.60" | 12.67" | +1.8 mm |
| Approved rim width | 5–6.5" | 5–6" | overlap 5–6" |
Both are approved on a rim 5" to 6" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 185/70R15. Fit 175/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.4 mph | +0.4 |
| 90 mph | 90.5 mph | +0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,055 on the road, so it under-counts by 55. At a true 70 mph the first turns 934 times a minute, the second 929.